Liquefaction of nitrogen in regasification of liquid methane



May 18, 1965 H. HASHEMlf-TAFRESHI LIQUEFACTION OF NITROGEN IN REGASIFICATION OF LIQUID METHANE Filed June 16. 1960 5 Sheets-Sheet 2 Qw km.

May 18, 1955 H. HAsHl-:Ml-TAFREsl-u 3,183,677

LIQUEFAC'IION OF NITROGEN IN REGASIFICATION OF LIQUID METHANE Filed June 16. 1960 3 Sheets-Sheet 5 United States Patent O 3,183,677 LIQUEFAC'IION 0F NITRGEN IN REGASHFI- CATION F LIQUID METHANE Hadi Hashemi-Tafreshi, London, England, assigner to Couch International Methane Limited, Nassau, Bahamas, a corporation of Bahama Filed June 16, 1960, Ser. No. 36,643 13 Claims. (Cl. 629) This invention relates to producing liquid nitrogen in the course of the regasification of liquid methane. The liquid methane may come from any source and in particular may be liquefied natural gas which consists primarily of methane. In this specification, therefore, references to methane mean pure methane or any gas mixture, the major ingredient of which is methane.

The invention provides a method of producing liquid nitrogen in the course ot the regasification of liquid methane which comprises (a) Cooling gaseous nitrogen by indirect heat exchange with compressed liquid methane to produce compressed gaseous methane,

(b) Comprising the cold gaseous nitrogen from Step (a),

(c) Condensing the cold compressed gaseous nitrogen from step (b) by indirect heat exchange with liquid methane to produce compressed liquid nitrogen and gaseous methane, and

(d) Heating the compressed gaseous methane from step (a) and expanding it to produce energy.

The energy produced in step (d) of the above method (which method is hereinafter sometimes called Method A) may be used to drive the compressors used in the process so that the only external source of power is the heat put into the system in step (d). This may be provided very cheaply from waste hot water. The whole of the refrigeration necessary to produce the liquid nitrogen in this method may be obtained from liquid methane so that no other source of refrigeration is required. The compression power required in the method is reduced to a minimum by ensuring that the inlet temperatures to the various compressors are as low as possible and in any case not higher than about 200 F.

It will be appreciated that the various temperatures and pressures employed in the method of the invention will be inter-related. Typical temperature and pressures are given in the more detailed description hereinafter appearing. However, generally the nitrogen will be cooled to 200 F. to 250 F. in step (a).

The gaseous nitrogen used as a feed in Method A may come from any suitable source; for example, it may be a Iby-product from a separate air rectification plant. Preferably, however, it comes from an air rectification plant built into, and operating in conjunction with, the liquid methane regasication plant. A suitable method for operating such a combined plant comprises (a) Cooling compressed gaseous nitrogen taken overhead from an air rectication column by indirect heat exchange with compressed liquid methane to produce compressed gaseous methane,

(b) Further compressing the cold gaseous nitrogen from step (a),

(c) Condensing the cold compressed gaseous nitrogen from step (b) by indirect heat exchange with liquid methane to produce compressed liquid nitrogen and gaseous methane,

(d) Cooling the air stream to the air rectier by indirect heat exchange with compressed liquid methane to produce compressed gaseous methane,

(e) Heating the compressed gaseous methane from steps (a) and (d) and expanding it to produce energy,

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(f) Further cooling the cold air from step (d) by heat exchange with cold nitrogen and/ or oxygen from the air rectifier and passing the further cooled air to the air rectifier from which gaseous nitrogen is taken oft as an overhead stream,

(g) Reducing the pressure on the liquid nitrogen from step (c) to produce liquid nitrogen at a lower pressure and cold gaseous nitrogen, and

(h) Returning some of the liquid nitrogen from step (c) and/ or (g) to the air rectifier as reflux,

The cold gaseous nitrogen from step (g) may join that coming overhead from the air rectifier and/or may be circulated through to coils of a reboiler in the air rectifier. Step (g) may be carried out in stages, for example, in two or more expansion chambers.

Again in this method (hereinafter sometimes called Method B), the compression power required is derived from energy from step (e) and all the refrigeration is obtained from the liquid methane so that the only external source of power required is the heat input in step (e).

It will be apparent that the method produces from liquid methane and air as starting materials gaseous methane, the product from steps (a) and (e), and liquid nitrogen, the product from step (g), the only external source of power being the heat input into step (e). The method also yields gaseous or liquid oxygen as will be described in more detail hereinafter.

lf necessary, further energy can be obtained by using one or more external energy-producing cycles in heating up the compressed gaseous methane, for example as required in step (d) of Method A or in step (e) of Method B. The external energy-producing cycles are those of the type described in British patent specication 814,209 by heating up the compressed gaseous methane by means of an external energy-producing cycle in which heat is supplied to the compressed gaseous methane from a working medium of higher boiling point by heat exchange in a condenser in which the working medium is liquefied and the Working medium is then brought to a higher pressure, vaporised at the higher pressure, expanded in an engine to produce energy and then recycled to the condenser.

The method of producing liquid nitrogen in the course of the regasification of liquid methane which is the sub- .ject of the invention Will now be described with reference to the accompanying drawings, in which FIGURE 1 is a ow diagram of a simple process operating in accordance with Method A, and

FIGURE 2 is a flow diagram of a process operating in accordance with Method B, and

FIGURE 3 is a ow diagram of a modification of the process illustrated in FIGURE 2.

Referring to FIGURE 1, liquefied natural gas at its boiling point at atmospheric pressure, viz. 258 F., enters the system by a conduit 1 and is divided into two streams, one going through conduit 2 and the other through conduit 3. The stream passing through conduit 2 is compressed in compresser 4 to 1500 p.s.i.a. lts temperature will then to about -248 F. This compressed liquefied natural gas is then used to cool compressed gaseous nitrogen in heat exchangers 5 and 6. A further part of the compressed liquid natural gas is used to cool the nitrogen down to -240 F. before it is compressed. This it does in heat exchanger '7. The three streams of natural gas which have passed through the heat exchangers 5, 6 and 7 meet in conduit 3 and pass through a heat exchanger 9 where they are further warmed to 100 F. and the resulting natural gas which is still at 1500 p.s.i.a. is expanded in expander 10 down toa pressure of 390 p.s.i.a. with the production of energy. The natural gas then passes through a further heat exchanger 11 in which its temperature is raised to F. It is then passed through a further expander 12 in which its pressure is reduced to 115 p.s.i.a. and its temperature to substantially ambient with the production of energy. The eiiiuent from expander 12 is suitable for feeding into a gas pipeline.

The source of heat in heat exchangers 9 and 11 may be a waste hot water supplyV or may be der-ived from an external energy-producing cycle of the previously described type for example a propane cycle.

The stream of liquefied natural gas passing through conduit 3 is evaporated in heat exchange-r 13 in liquefying compressed nitrogen. The natural gas at atmospheric pressure may be pumped up to pipeline pressure of 115 p.s.i.a. and joined with the efiiuent [from expander 12 if so desi-red. l

The nitrogen stream at atmospheric pressure enters the system via conduit 14 and is cooled in heat exchanger 7 to 240 F. It is compressed in compressor 1S to 82 p.s.i.a. and again cooled to 240 F. in heat exchanger 6. It is then compressed to 300 p.s.i.a. in compressor 16 and again cooled to 240 lF. in heat exchanger S. It then passes -through heat exchanger 13in i which it is liquefied to produce liquid nitrogen at 250 F. and under a pressure of 300 p.s.i.a. If the liquid nitrogen is required at atmospheric pressure, it can be fed into one or more expansion chambers to Vlet the pressure down, the resulting cold gaseous nitrogen which dashes oli being fed back into the system, if desired.

The power required in compressors 4, and 16 can be derived from expanders 10 and 12 augmented if necessary by energy produced in external energy-producing cycles attached to heat exchangers 9 and 11. Similarly, energy required for pumping up gaseous methane at atmospheric pressure from the eliluent of conduit 3 to a pipel-ine pressure of 115 p.s.i.a. may also be derived from these energy-producing sources within the cycle.

It will be appreciated that the process described in FIGURE 1 requires a source of nitrogen and that the most convenient source of nitrogen is an .airrectifien It is possible to so design the air rectiiieation process that it can be fitted into the process of FIGURE 1 and operated without any external power apart from waste heat or refrigeration other than that provided by the liqueied natural gas. Such a process will now be described with reference to FIGURE 2.

Liquid methane at 258 F. and atmospheric pressure enters the system via conduit 20 and is divided into two streams. One of these streams in conduit Z1 is compressed in compressor 22 to 1500 p.s.i.a. and the cornpressed methane is again divided into two streams, one of which passes through conduit 23 and the other through conduit 24. That passing through conduit 23 is used to cool, in heat exchanger 24, the incoming air stream to the air rectifier, in the course of which operation its temperature is raised to 40 F. and it is gasified. The compressed liquid methane in conduit 24 is used in heat exchanger 26 to cool compressed gaseous nitrogen, in the course of which the compressed liquid methane is gasied :and its temperature raised to 100 F. The

temperature of this compressed gaseous methane is raised to 50 F. in heat exchanger 27 which is the condenser of an external energy-producing cycle to be described later. Some of the compressed gaseous methane at 100 F. may be bled oit before heat exchanger 27 and led to an appropriate point in heat exchanger 25 via conduit 62.

After leaving heat exchangers 25 and 27, the comressed gaseous methane streams are joined `in conduit 28 and heated by means of hot water in heat exchanger 29 to 160 F. The gaseous methane at 160 Fand 1500 p.s.i.a. is expanded in expander 30 to a pressure of 400 p.s.i.a. with the production of energy. It is then heated by means of hot water in heat exchanger 31 to 160 F.

again and again expanded in expander 32 to a pipeline pressure of 115 p.s.i.a. with the production of energy.

-T he external energy-producing cycle of which heat exchanger 27 is the condenser is a closed propane cycle which operates as follows.A Gaseous Vpropane ata pressure of l5 p.s.i.a. and a temperature of V 43 F. is condensed in heat exchanger 27 to liquid propane which is then compressed in compressor 34 toa pressureof 400 p.s.i.a. The compressed liquid propane coming from compressor 34 iiows through a heat exchanger 35 in which it is heated by means of warm water up to 200 F. and vaporised. The Warm compressed propane vapour is then expanded in expander 36 to a pressure of 15 p.s.i.a. with the production ofy energy. The cycle is repeated continuously.

The second part of the liquid methane entering the systemyia conduit 20 iiows through heat exchanger 37 in which it is vaporised at atmospheric pressure while cooling compressed nitrogen and liquefying it. The gasiiied methane may be pumped up to pipeline pressure of 115 p.s.i.a. in compressor 3S. n

The Vforegoing described the ilow of methane in the process in FIGURE 2. The-re will now be described the iiow of air, oxygen and nitrogen in the process of FG- URE 2. `Air at atmospheric pressure is fed through conduit 39 to compressor 40 inwhich `itfis compressed to 18 p.s.i.a. The slightly compressed air passes through heat exchanger25 inwhich it is cooled from 60 F. to 240 F. by means of compressed liquid methane and 'cold oxygen from the air rectier. The `air then passes throughra `fur-ther heat exchanger 41 in Vwhich it is cooled to 290 F. by cold nitrogen and cold oxygen from the air rectifier. The air at 290 F. enters the middle of the column of the air rectilier 42., In the rair rectiier 42, liquid oxygen collects at uthe bottom and substantially pure nitrogen goes overhead via conduit '43.; The nitrogen stream in conduit 43 is at a temperature of 320 F. and a pressure of 14.7 p.s.i.a. It passes through heat exchanger 44 in which it cools liquid nitrogen returning as reux to the air rectitier and is vitself heated to 295 F. It thenfiows through heat exchanger 41 and then to compressor 45 in which it is compressed to 80 p.s.i.a. The compressed nitrogen then passes through heat exchanger 26 in which it is cooled to 240 F. while gasifying compressed liquid methane. The nitrogen stream at 240 F. is lthen compressed in compresser 46 to a pressure of 320 p.s.i.a. and is again passed through heat exchanger 26 to cool it to 240 F. The compressed gaseous nitrogen at 240 F. and 320 p.s.i.a. then passes through heat exchanger 37 in which it is -liqueiied in regasifying methane. Part of the liquefied nitrogen is returned to the Yair rectifier 42 as reux via conduit 47, heat exchangers 41 Vand 44 and conduit 48. Part, of it may also be=fed to :expansion chamber 53 via conduit 47, y'heat exchangers 41 and 44 and conduit 56.

Another portion of the liquefied nitrogen is fed via conduit 49 to expansion chamber 50 in which the pressure is let` down to p.s.i.a. resulting in a iiashing oit of nitrogen gas at 290 F. which is lfed via conduit 51 to the coils of the reboilei 59 in the bottom of the air rectifier, in which coils it is liquefied and returned to the liquid nitrogen conduit 47 via conduit V60'.

Liquid nitnogen at 290 F. collects in the bottom expansion chamber 50 and is fed from there via conduit 52 to -a second expansion chamber 53 is lwhich the pressure is let down. tto 15 p.s.i.a. resulting in the production of nitrogen `gas at 320 F. which is taken olf via conduit 54 to join 'the nitrogen stream lfrom the top of the lair rectiiier =in conduit`43. In expansion chamber 53, fliquid nitrogen at 320 4and v15p.s'.i.a. collects and is drawn off `as Ean end product via conduit 55. Some of the liquid nitrogen from expansion chamber 50 may be led via conduit 61 back to conduit A47 and hence back to the yair rectifier as redux.

Liquid oxygen may be drawn loit from the -air rectifier via valve l57 ibut the hulk of the oxygen in this process leaves the air rectifier las' la saturated vapour via conduit 58 and is used in heat exchangers 41 and 215 to cool the incoming air.

IE it is desired to increase liquid Oxygen production as ian end product, the use of cold oxygen vapour in heat exchangers `@t1 `and -25 can be dispensed with so that a larger proportion of :liquid oxygen can he taken oli via valve 57. In this case, some or all of the nitrogen will have to be bled out of the system as a gas after heat exchange in heat exchanger 25 or some external power will be required. This process is now described with reference to `FIGURE 3.

Liquid methane at 258 F. and atmospheric pressure enters the system via conduit 70 and is divided into two streams. One of these streams in conduit 71 is compressed in compressor 72 lto 1500 p.s.i.'a. and the compressed methane is again divided into two streams, one of which passes ythrough conduit 73 and the other through conduit 74. That passing through conduit 73 is used to cool, in heat exchanger 75, the incoming air stream to the air rectifier, in the course of which operation i-ts temperature is raised to 40 F. `and it is gasified. The compressed Iliquid methane in conduit 74 is used in heat exchanger 75 to cool compressed gaseous nitrogen, in the course of which the compressed liquid methane is gasied and its temperature raised to 100 F. Some of this compressed gaseous methane may be bled via conduit 77 to an appropriate point in heat exchanger 75, but the rest joins the compressed gaseous methane stream from heat exchanger 7S in co-nduit 7S and is then heat-ed by means of hot water in heat exchanger 79 to 160 F. The gaseous Imethane at 160 F. and 1500 p.s.i. a. is expanded in expander `80 to a pressure of 400 p.s.i.a. with the production of energy. It is then heated 'by means of hot water in heat exchanger 81 to 160 F. aga-in .and again expanded in expander 82 to the pipeline pressure of 115 p.s.i.a. with the production of energy.

The second part of the liquid methane entering the system via conduit 70 flows through heat exchanger 87 in which it is vaponised lat atmospheric pressure While cooling compressed nitrogen and liqueiying it. The gasiiied methane may be pumped up to pipeline pressure `of 115 p.s.i.a. in compressor `88.

The foregoing describes the cli-ow of methane in the process in IFIGURE 3. There will now he described the how of air, oxygen Iand nitrogen in the process of FIGURE `3. Air at yatmospheric pressure is fed through conduit `S9 through hea-t exchanger 75 in /which it is cooled from 60 F. to 240 F. by means of compressed liquid methane and cold nitrogen from the air rectifier. The :air then passes through la further heat exchanger `911. in which it is -cooled to 290 F. by further cold nitrogen from the `air rectifier. The air at 290 F. enters the middle vof the column of the lair rectiier 92. In the air rectier 92, liquid Oxygen collects at the bottom and substantially pure nitrogen goes ov-erhead lvia conduit 93. The nitrogen stream in conduit 93 is `at a temperature of 320 F. `and a pressure of 14.7 p.s.fi.=a. It passes through heat exchanger 94 in which it cools liquid nitrogen returning as reflux to the `air rectifier and lis itself heated to 295 F. It then .iiows through heat exchanger 91, after which it is divided into two streams, part passing through hea-t exchanger 75 and hence to atmosphere and par-t pass-ing to compressor 95 in which it is compressed to 80 p.s.i.a. The compressed nitrogen then passes through heat exchanger 76 in which it is cooled 4to 240 F. while gasifying compressed liquid methane. The nitrogen stream at 240 F. is then divided into two streams, one of which is compressed lin compressor 96 to a pressure of 320 p.s.i.a., is `again passed through heat exchanger 76 6 to cool it to 240 F., `after Which it vpasses through heat exchanger 187 in which it is liqueiied in regasifying methane. The other stream is returned :to the -reboiler 109 of the air rectiiier `92 via conduit 97, heat exchanger 91 and conduit 1:10.

The nitrogen liquefied in heat exchanger 87 is yfed via conduit 99 to expansion chamber `100 in which the pressure is let down to 76 p.s.i.a. resulting in a flashing off of nitrogen gas at 290 F. which is fed via conduits :101 and 110 to the coils of the reboiler i109 in the bottom or" the air rectier, in which coils it is liqueiied and yfrom which it is returned to the upper part of the air rectier :as reflux via conduits 108 and 98. Liquid nitrogen at 290 yF. collects in the bottom of expansion chamber 100 and is ifed lfrom there via lconduit 102 to a second expansion chamber -103 in which the pressure is let down to l5 pls-ita. resulting in the produc-tion of nitrogen gas at 320 lF. which is taken off via conduit les to join the nitrogen stream from the top of the lair rectifier in 'conduit 93. :In expansion chamber 103, `liquid nitrogen :at 320 F. and l5 p.s.i.a. collects and is 4dr-awn of as an end product via conduit 105. ySome of the liquid nitrogen from expansion chamber may be led via `conduit fidi back to conduit 98 and hence back to the yair rectifier as reiiux.

Liquid oxygen is drawn oii from the air rectifier via conduit 107 as an end product. Where the methane has to ybe reformed by an oxidation process, this oxygen is readily `available for this purpose.

I claim:

I. A :method of producing liquid nitrogen in the course of the regasiication of liquid ymethane which comprises (a) cooling gaseous nitrogen by indirect heat exchange Iwith compressed liquid methane to produce compressed gaseous methane,

(b) compressing the cold gaseous nitrogen trom step (c) condensing the cold compressed gaseous nitrogen from step (b) 'by indirect heat exchange with liquid methane to produce lcom-pressed liquid nitrogen and gaseous methane, and

(d) addition-ally heating the compressed gaseous methane from step (a) to further raise its temperature `and expanding it to produce energy.

2. A method as claimed in claim 1 in which the energy produced on expanding the compressed gaseous methane is used to drive the compressors used in the process.

3. A method as claimed in claim 2 in which the inlet temperatures of the various compressors is not higher than about 200 F.

4. A method as claimed in claim 1 in which the gaseous nitrogen is cooled to 200 F. to 250 F. by indirect heat `exchange with compressed liquid methane.

5. A method for producing liquid nitrogen in the course of the regasication of liquid methane in which gaseous nitrogen is supplied from an air rectification plant operating in conjunction with the liquid methane regasiication plant, said method comprising (a) cooling compressed gaseous nitrogen taken overhead from an air rectification column by indirect heat exchange with compressed liquid methane to produce compressed gaseous methane,

(b) further compressing the cold gaseous nitrogen from Step (a),

(c) condensing the cold compressed gaseous nitrogen from step (b) by indirect heat exchange with liquid methane to produce compressed liquid nitrogen and gaseous methane,

(d) cooling the air stream to the air rectifier by indirect heat exchange with compressed liquid methane to produce compressed gaseous methane,

(e) heating the compressed gaseous methane from steps (a) and (d) and expanding it to produce energy,

7 (f) further cooling the cold air from step (d) by `hea-t exchangewith cold nitrogen from the air rectiiier and passing the further cooled air tothe air rectilier from which gaseous nitrogen is `taken olf as an overhead 7. A process as claimed in claim 6 in which the cold y 11. A method as claimed inclaim 5 in which some of the liquid nitrogen from step (g) is returned to the air rectifier as reflux.

12. A method of producing liquid nitrogen in the course of the regasication of liquid methane Whichcomprises (a) cooling gaseous nitrogen by indirect heat exchange with compressed liquid methane to produce compressed gaseous methane,

(b) compressing the cold gaseous nitrogen from step (c) condensing the cold compressed gaseous nitrogen from step (b) by indirect heat exchange with liquid methane to produce compressed liquid nitrogen and gaseous methane,V heating the compressed gaseous methane from step (a) and expanding it to produce energy, and

(d) obtaining further energy by heating Vup the compressed gaseous methane by means of an external energy-producing cycle in which heat is supplied to the compressed gaseous methane from a working medium of higherV boiling point by heat exchange in a condenser in which the working medium is liqueed and the working medium is then brought to a higher pressure, vaporised at the higher pressure, expanded in an engine to produce energy and then recycled to the condenser.

13.v A method as claimed in claim 12 wherein the external energy-producing cycleris a propane cycle.

References Cited by. the Examiner UNITED STATES PATENTS 73 6 ,73 6 9/ 32 Luces-Girardville. 2,601,009 6/52 Swearingen. 2,799,997 7/57 Morrison. 2,875,589 3/59 Horn. 2,937,504 5/ 60 Riediger 62-53 2,9 75 ,604 3 61 McMahon 62-9 X f YFoRnrGN PATENTS 804,944 `1 1/5 8 Great Britain.

NORMAN YUDKOFF, Primary'Exuminer. Y ROBERT A. OLEARY, Examiner. 

1. A METHOD OF PRODUCING LIQUID NITROGEN IN THE COURSE OF THE REGASIFICATION OF LIQUID METHANE WHICH COMPRISES (A) COOLING GASEOUS NITROGEN BY INDIRECT HEAT EXCHANGE WITH COMPRESSED LIQUID METHANE TO PRODUCE COMPRESSED GASEOUS METHANE, (B) COMPRESSING THE COLD GASEOUS NITROGEN FROM STEP (A) (C) CONDENSING THE COLD COMPRESSED GASEOUS NITROGEN FROM STEP (B) BY INDIRECT HEAT EXCHANGE WITH LIQUID METHANE TO PRODUCE COMPRESSED LIQUID NITROGEN AND GASEOUS METHANE, AND (D) ADDITIONALLY HEATING THE COMPRESSED GASEOUS METHANE FROM STEP (A) TO FURTHER RAISE IT TEMPERATURE AND EXPANDING IT TO PRODUCE ENERGY. 